A major concern of near field sensors is the distance between the substrate under investigation and the near field sensor. This is an unavoidable consequence of the working principle underlying near field optics, as the advantage of the near field sensor lies in the sensitivity of the near field sensor to evanescent modes that exist only close to the surface of the substrate.
These evanescent modes die out exponentially fast over a fraction of a wavelength as the near field sensor recedes from the sample.
Thus, at visible to deep ultra violet (UV) wavelengths, near field sensor height control at the range of 1 nanometer scale is required. In contrast, in ordinary high numerical aperture optical settings, the typical height tolerance is the depth of focus, of order of the wavelength.
A significant caveat of traditional height measurement techniques (such as optical interferometry and optical wave-front sensing) is their inherent sensitivity to both the material and the three dimensional pattern geometry of the measured substrate.
When the material and/or three dimensional pattern geometry change across the substrate, the amplitude and phase of reflected waves will change accordingly, and will affect, in turn, the diffraction pattern used for height measurement. Analogous arguments pertain to electrical capacitive measurements.
There is a growing need to provide robust height measurements.